@juiceboks: Dark Knight Returns Part 2. Superman was sucked dry by the nuclear missile. In ep. Frogs of War 1(omniverse) NRG was able to get out of his suit without help of any outer agent. If he gets out, he can be a problem. None of Supes attack will affect him. His freezing breath will merely slow him down(since he can heat up to millions of celcius).
And Chromastone CAN beat Clark. Chromastone absorbs energy and fires it in different forms(sometimes laser, sometimes simply conc. solar beam..) He can easily manipulate solar beam to red sun radiation. Then defeating Superman will be a child's play(not really). Also if you've seen Vengeance of Vilgax, you'd know that Diamond head defeated Vilgax very easily and that Vilgax took had power of Ultimos and other most powerful 9 heroes in him, who by the way looked pretty powerful. Vilgax was able to defeat 60 ft grown Humagosaur in mere seconds. He was
as powerful as Darksied himself. And Diamond head finished him off in less than a minute.
Another alien which can most definitely defeat Superman is Gravattack. He is Graviton and more powerful since he doesn't have the limitations of being human. He is durable as he was able to shrug off some mighty blows before Ben figured out his powers. He can keep applying pressure(actually force) on Superman until he breaks literally or make him feather light. Its hard to fight when you are getting blown away by the wind.
Clockwork can also defeat Superman. Superman, very slowly, but ages. So yeah Clockwork can kill him.
@allstarsuperman About alien X. Serena and Bellicus are proud. According to them, they are the ultimate power being(which they are). They don't actually care about who's good and who's evil(AF episode Ben=X+2- They denied to wipe Incursians out of existence when they had destroyed a planet and goint to destroy earth.) Ben hurt their ego and they recklessly let him manipulate them. Also in UA episode Forge of Creation Paradox hinted that Ben will be able to use Alien X without any difficulty.